public static IDataflow <Either <TLeft, IGroupedDataflow <TKey, TRight> > > GroupBySafe <TLeft, TRight, TKey>(
this IDataflow <Either <TLeft, TRight> > source, Func <TRight, TKey> keySelector)
{
return(source
.GroupBy(item => item.IsRight)
.SelectMany(
group => group.Key
? group
.Rights()
.GroupBy(keySelector)
.Select(Right <TLeft, IGroupedDataflow <TKey, TRight> >)
: group
.Lefts()
.Select(Left <TLeft, IGroupedDataflow <TKey, TRight> >)));
}